Semiconductor switches, in particular power transistors for switching large currents and voltages, often require special driver circuits for generating the control signals which control the switching-on and switching-off of the power transistors. Power transistors are often used as semiconductor switches in transistor half-bridges which often comprise only n-type transistors (for example n-channel MOSFETs, n-channel IGBTs) in the case of power applications. The reason for this lies in the electrical properties which are better than those of p-type transistors.
Transistor half-bridges comprise a high-side transistor and a low-side transistor. In this case, the high-side transistor is connected between an upper supply potential and the output of the half-bridge, and the low-side transistor is connected between the output of the half-bridge and a lower supply potential. High-side transistors require control signals which are not based on a constant reference potential, for example the ground potential, but rather on the output of the half-bridge which is generally connected to the emitter or source of the high-side transistor. The same applies to p-type low-side transistors. The need for floating control signals makes it difficult to supply voltage to driver circuits for high-side transistors. This is the case, in particular, when the power semiconductor transistors have to take up reverse voltages of several hundred volts but the driver circuit itself is not supposed to have any high-voltage-resistant semiconductor components or is intended to have as few high-voltage-resistant semiconductor components as possible for reasons of space and on account of the production technology.
Different possibilities for supplying driver circuits for high-side transistors in a floating manner are known. One of them is to use a bootstrap circuit. A bootstrap circuit is understood as meaning a combination of a diode and a capacitor, the driver circuit being supplied either from the capacitor or directly from an auxiliary voltage source via the diode. In the latter case, the capacitor is also charged from the auxiliary voltage source via the diode. The charge stored in the capacitor is then supplied to the driver circuit again depending on the switching state of the transistor. Specially clocked transistors may also be used instead of diodes.
Another possibility is to use a self-clocked charge pump. A charge pump is understood as meaning an arrangement which comprises a clock generator, switches and capacitors and allows a capacitor to be charged at a voltage source, to then be disconnected from this source and to be discharged into a load. In this case, the switches may be transistors or diodes, transistors having to be used at at least one point. In the text below, a charge pump is always understood as meaning a self-clocked charge pump whose clock is independent of the drive signal for the power transistors.
The sole use of a bootstrap circuit for supplying the driver circuit for a high-side power transistor often does not suffice since capacitors of an appropriate size are often not able to store sufficient charge to be able to maintain the supply for the driver circuit for a sufficient amount of time. It is often not possible to increase the size of the capacitor for reasons of space. For this purpose, the bootstrap circuit may be supplemented with a charge pump which keeps the capacitor, which is charged by the diode of the bootstrap circuit, in a charged state. In this case, the charge pump may be dimensioned in such a manner that only the charge which has flowed away through the driver circuit during a clock cycle (of the charge pump) is replaced again by the charge pump. However, the clock generator of such a charge pump likewise requires a floating voltage supply. In a simple case, the voltage supply for the clock generator can be directly derived from the upper supply potential with the aid of a series circuit comprising a resistor and a zener diode. However, this presupposes a high-voltage-resistant resistor, which may be undesirable for reasons of production technology. In addition, a considerable power loss is consumed in the resistor. If the supply current of the driver exceeds an order of magnitude of 1 mA, for example, such a solution is unattractive despite its simplicity.
There is a general need to provide a circuit arrangement for providing a supply voltage for a transistor driver circuit, which manages, as far as possible, without high-voltage-resistant semiconductor components, in which the power loss consumed is as low as possible and which is able to supply the driver circuit without time limitations.